Cladding of metals

ABSTRACT

This invention relates to articles for use at high operating temperatures (1,100*-1,500* C.) and comprising a core made from a refractory metal or alloy and clad with a sheath of a platinum group metal or alloy based on at least one platinum group metal. Such articles in the form of stirrers, crucibles, spinning dies and the like have particular application in the glass industry.

United States Patent Selman et al.

[ 51 Apr. 25, 1972 CLADDING OF METALS inventors: Gordon Leslie Selman,High Wycombe;

Alan S. Darling, Northwood, both of En- [56] References Cited UNITEDSTATES PATENTS 2,681,876 6/1954 De Santis et al. ..29/l98 X 2,497,0902/1950 Miller et al. ..250/27.5 2,947,1 14 8/1960 Hill ..49/53 2,536,6731/1951 Widell ..250/27.5

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-E. L. WeiseAttorney-Hofgren, Wegner, Allen, Stellman & McCord [57] ABSTRACT Thisinvention relates to articles for use at high operating temperatures(1,100-1,500 C.) and comprising a core made from a refractory metal oralloy and clad with a sheath of a platinum group metal or alloy based onat least one platinum group metal. Such articles in the form ofstirrers, crucibles, spinning dies and the like have particularapplication in the glass industry.

17 Claims, No Drawings CLADDING F METALS This is a continuationin-partof application Ser. No. 631,592 filed Apr. 18, 1967, now abandoned.

This invention relates to the cladding of refractory metals withplatinum group metals or alloys based on at least one metal of theplatinum group. Such alloys will be referred to herein as platinum basealloys.

The invention is particularly but not exclusively applicable to thecladding with the aforesaid metals or alloys of articles which are usedin the glass industry, such as, s'tirrers, crucibles, spinning dies,tube fabricating dies and the like.

Of the platinum group metals we have found platinum, rhodium, palladiumand iridium particularly suitable cladding metals. 0f the alloysconsisting predominantly of one or more platinum group metals,satisfactory results have been obtained from the following claddingalloys; rhodium/iridium; platinum/iridium; palladium/iridium, andplatinum/rhodium/iridium.

Such articles are required, in use, to withstand the action of moltenglass at temperatures at least as high as 1,100" C. and sometimes ashigh as 1,500 C. Further, they are generally required not to introducemeasurable quantities of impurities into molten glass with which theycome into contact.

Platinum metal and certain rhodium/platinum alloys, for example, resistattack by molten glass and also have good hot strengths. It would thusbe possible to make articles of the kind mentioned previously fromplatinum or rhodium/platinum alloys. The cost would, however, beprohibitive. A stirrer is typically in the form of an inverted T inwhich the vertical shaft is about 6 feet long and 1% inch diameter andthe horizontal shaft is 4 feet long and 1% inch diameter. It hasaccordingly become the practice to fabricate articles such as stirrersfrom a core of a metal such as molybdenum which is very much cheaperthan platinum and which has good hot strength but which would becompletely and rapidly oxidized when exposed to air at the temperatureof molten glass, and to enclose or c1ad" this core in a thin,continuous, closely fitting sheath of platinum metal or platinum basealloy. It is found in practice that such an article has the advantagesnot only of being cheaper than a corresponding article made wholly ofplatinum but also of being stronger at temperatures in the region of1,400 C. since molybdenum, at this temperature, is stronger thanplatinum.

The platinum or rhodium/platinum sheath is usually applied by fittingsuitably shaped pieces of platinum of platinum/rhodium alloy sheet roundthe article to be clad and then welding these pieces together so as toform a closely fitting cladding or covering for the core. The claddingis generally provided with one or more small exhaust tubes whichcommunicate with the interfacial space between the cladding and the corebut is otherwise gas-tight.

The final stage in the cladding of the core involves a reduction in thepressure of the fluid (e.g. gas, vapor) in the interfacial space via theexhaust tube(s) down to a low value following which the tube(s) arepinch sealed. The pressure in the interfacial space is reduced in thisway so as to minimize the oxidation of the molybdenum core in serviceand to improve the fit of the cladding. The value to which the pressureof the fluid in the interfacial space is reduced will depend inter aliaon the geometry of the core and may be within the range of a few micronsto 2 mm. of H,.

Molydenum articles clad in this way were, however, found to be prone toearly failure in service. In general the failure takes the form of afracture or split in the cladding which permits molten glass'to reachthe underlying molybdenum core. It was originally thought that suchfailures were due to the difl usion of molybdenum from the core into theplatinum cladding at the region of contact between the cladding and thecore. Attempts were, therefore, madeat least to reduce this effect byinterposing a barrier layer of some refractory material such 'as aluminabetween the core and the cladding so as to prevent direct contacttherebetween. Some improved results were obl,250 C. when the alumina wasflame sprayed on to the core but, although this procedure was found toprolong the operating life of a stirrer, the improvement was not marked.

We have now discovered that, although the failure of the platinumcladding is due to diflusion of the molybdenum into the platinum, themolybdenum is transferred to the platinum cladding in the vapour phaseas one or more oxides of molybdenum. It is believed that the trioxide ispredominant. At the temperatures at which a stirrer is used in the glassindustry, the molybdenum oxides formed by the interaction of the corematerial and any residual oxygen in the interfacial space are tained atthe lower operating temperatures in the region of volatilized and, whenthey contact the inner surface of the platinum cladding, they arereduced to the metal, which then alloys with the platinum. The oxygenliberated in this way is then available to oxidize more of themolybdenum core so that the action is continuous and we have found thatthe inner regions of the cladding begin to breakdown, thus leading toultimate failure.

We have also found that the operating life of a core clad with platinumor with a platinum base alloy can be greatly improved if the core ismade from a metal selected from the group consisting of niobium,tantalum, niobium-tantalum alloys, niobium-chromium alloys,tantalum-chromium alloys and niobium-tantalum-chromium alloys.

The oxides of niobium and tantalum are much less volatile than theoxides of molybdenum at the operating temperatures of l,l00 1,400" C. orl,500 C. The platinum (or platinum base alloy) cladding is affected bycores of these metals much less, and, consequently, the operating lifeof the article is considerably increased.

For example, an article consisting of a molybdenum core loosely sheathedwith 0.020 inch thick platinum and in which the interfacial volume wasnot fully evacuated and thus contained some residual oxygen, was foundto be protected for about 400 hours at an operative temperature of 1,400C. in air. A similar sheath on a core of niobium under the sameconditions provided protection for 1,000 to 2,000 hours, thus showingthe advantages provided by the non-volatility of niobium oxide.

However, even with niobium and tantalum core materials there must stillbe certain points of contact between the sheath and the underlying core.At these points the core metal tends to diffuse through the sheath and,upon reaching the exterior, oxidation again occurs leading to suddenfailure of the sheath. In the case of articles in contact with moltenglass this is unacceptable as it leads to discolouration of the glass.

Very much longer life times than can be obtained with niobium andtantalum core materials therefore necessitate th use of alternativematerials.

Other metals which might be considered, such as titanium, zirconium andvanadium do not form volatile oxides but their melting points are toolow to allow of their effective use. They are weak mechanically at theaverage operating temperatures of 1,350 1,400 C. and moreover they havethe undesirable tendency to alloy with the platinum or platinum basedsheath to form diffused alloy layers, intermediate phases, and in someinstances, phases with melting points lower than the temperature atwhich the component is intended to operate.

As previously mentioned, an arrangement using a molybdenum core has ashorter operating life than is desirable, and attempts to prolong theoperating life by reducing the pressure in the interfacial spacesbetween the core and the sheath have not up to the present time met withsuccess.

However, our investigations indicate that a further reduction of thepressure in the interfacial space may reduce to an even greater degreethe migration of the molybdenum (in the form of oxide) to the claddingmaterial.

We have found that the application to the molybdenum or tungsten core ofa getter in the form of a refractory coating greatly aids evacuation ofoxygen from the interfacial volume and increases to aconsiderable extentthe operating life of a molybdenum/tungsten core platinum/platinum alloysheath arrangement.

Suitable getter materials are metals such as zirconium, tantalum,niobium, titanium, vanadium or hafnium. These metals may also be appliedin smaller quantities as dilute alloys of molybdenum or platinum.

As direct contact between the core or layer of getter material has beenfound to be undesirable, the invention also includes the use of abarrier layer between the core and getter layers and the outer sheath ofthe article.

The barrier layer may comprise:

a. refractory oxides (i.e. oxides which are themselves refractory; notnecessarily the oxides of refractory metals;)

b. refractory carbides;

c. refractory nitrides (for example boron nitride and silicon nitride);

d. Refractory sulphides; and

e. other refractory compounds which are compatible at operatingtemperature with the two materials with which they come into contact.

The above items (a) (e) include the compounds of the rare earth metals.

According to a further feature of the present invention, therefore, anarticle intended to be used at high operating temperature, above l,000C. comprises:

a. a core comprising a refractory metal selected from the groupconsisting of molybdenum, tungsten and an alloy based on at least one ofsaid metals;

b. a getter" applied to the core in the form of a refractory coating;

c. a barrier layer applied to the getter comprising a refractorycompound selected from the group consisting of refractory carbides,silicides, borides, sulphides, nitrides and oxides, wherebyvolatilization of the oxides of said core material is prevented fromforming with an outer sheath an alloy of lower melting point than thatof the sheath, and

d. an outer sheath enclosing the core, "getter" and barrier layers andcomprising a material selected from the group consisting of the platinumgroup metals and an alloy based on at least one platinum group metal.

In a preferred embodiment of the invention, the pressure in theinterfacial space between core and sheath is within the range of a fewmicrons to 2 mm. of mercury.

The low oxygen pressure required to ensure the continued operation ofthe molybdenum cored component may be easily obtained by the use of agetter as detailed above. However, the use of these getters produce sucha low partial pressure of oxygen that the more usual refractory oxides,such as alumina, zirconia and thoria, for example, tend to decompose.The metal so released tends to alloy with the platinum of the sheath andthis again produces early failure of the component.

We have found that a barrier layer to which this does not apply iscomposed of magnesia. A preferred embodiment of the invention,therefore, is the use of magnesia as the barrier layer.

As an example of the invention a molybdenum or tungsten core may beflame-sprayed with zirconium metal. This results in a coating comprisinga mixture of zirconium metal, zirconium oxide and zirconium nitride. Inorder to prevent contact of a platinum sheath with zirconium metal,zirconia is then flame-sprayed onto the previously flame-sprayed layerof zirconium. The quantity of zirconium first sprayed is chosen so thatthe amount of zirconium available is only slightly greater than thatrequired to take-up the oxygen present in the interfacial space.Preferably, the zirconium oxide is porous to allow a rapid movement ofgas.

In carrying out an experiment to determine the effect of zirconium as agetter" material in platinum clad molybdenum, a core of molybdenum wassprayed with zirconium metal to prove a layer 0.003 inch thick. Thislayer was then oversprayed with zirconia to provide an outer layer 0.010inch thick. The composite core was then sealed into a platinum sheath0.020 inch thick without evacuation of the interfacial volume. The corehad a life of over 2,000 hours at a temperature of l,400 C.

If desired, zirconium or other getter may be incorporated within themolybdenum or tungsten core so as to form an alloy of for example 0.5 1.0 wt. percent zirconium/molybdenum.

Further, it will be appreciated that a composite core comprising acentral core of molybdenum having a first layer of titanium, zirconiumor vanadium as getter and finally an outer barrier layer formed from arare earth compound oxide with refractory properties, particularlymagnesia, may be used.

The getter may be plasmaor flame-sprayed in a layer uniformly over themolybdenum or tungsten surface of the core.

Alternatively it may be concentrated at a point in a fairly cool area ofthe core (e.g. 700 1,200 C.) so that it can absorb oxygen veryeffectively without coming into direct contact with barrier layermaterials at high temperature because such contact would lead todecomposition.

For example, in a molten glass stirrer the getter may be concentratedaround the upper part of the stem, above the surface of the moltenglass.

When tlameor plasma-sprayed magnesia is used as the barrier layer it maybe applied in combination with a small quantity of silica to assistadhesion to the core.

What is claimed is:

1. An article intended to be used at high operating temperatures abovel,000 C. comprising a molybdenum core, a zirconium metal coating on saidcore as a getter for oxygen, a barrier layer of the group consistingessentially of zirconia and magnesia on said zirconium metal getter, anda sheath enclosing the so coated core and consisting essentially of amaterial selected from the group consisting of a platinum group metaland an alloy having at least one platinum group metal.

2. An article according to claim 1 wherein said core comprises an alloyof said molybdenum with 0.5 1.0 wt. percent of zirconium.

3. An article according to claim 1 wherein the barrier layer comprisesmagnesia.

4. An article intended to be used at high operating temperatures abovel,000.C. comprising a core of a metal selected from the group consistingof niobium, tantalum, niobium-tantalum alloy, niobium-chromium alloy,tantalum-chromium alloy and niobium-tantalum-chromium alloy, a getterfor oxygen on the core consisting essentially of a refractory coating, asheath of a material selected from the group consisting of a platinumgroup metal and an alloy having at least one platinum group metal, and abarrier layer interposed between said core and said sheath comprising arefractory compound compatible with said core and said sheath at theoperating temperature, whereby volatilization of oxides of said corematerial is reduced and said core material is prevented from formingwith said sheath an alloy having a melting point below said operatingtemperature.

5. An article according to claim 4 further having an interfacial spacebetween the core and the sheath wherein the said space has a partialpressure of not more than 2 mm. of mercury.

6. An article intended to be used at high operating temperatures abovel,000 C. comprising: (a) a core consisting essentially of a refractorymetal selected from the group consisting of molybdenum, tungsten and arefractory alloy based on at least one of said metals; (b) a getter foroxygen on the core consisting essentially of a refractory coating; (c) abarrier layer on the getter consisting essentially of a refractorycompound selected from the group consisting of refractory carbides,silicides, borides, sulphides, nitrides and oxides, and (d) an outersheath enclosing the core, getter and barrier layers and consistingessentially of a material selected from the group consisting of theplatinum group metals and a platinum group metal alloy, volatilizationof the oxides of said core material thereby being prevented from formingwith said outer sheath an alloy of lower melting point than that of thesheath.

7. An article according to claim 6 having an interfacial space betweenthe core and the sheath wherein the said space has a partial pressure ofnot more than 2 mm. of mercury.

8. An article according to claim 7 wherein the getter is a materialselected from the group consisting of zirconium, tantalum, niobium,titanium, vanadium and hafnium.

9. An article according to claim 8 wherein the getter material ispresent in relatively small quantities as dilute alloys of molybdenum orplatinum.

10. An article according to claim 6 wherein the getter is in the form ofa plasma or flame sprayed coating.

11. An article according to claim 6 wherein the getter is concentratedin an area which, in use, is not subject to temperatures greater than1,200 C. so that the getter absorbs oxygen without contact with thebarrier layer, thereby avoiding decomposition of the sheath material.

12. An article according to claim 6 wherein the core material ismolybdenum alloyed with minor quantities of at least one of thematerials of the group consisting of titanium and zirconium.

13. An article according to claim 6 wherein the core material is alloyedwith at least one of the metals selected from the group consisting oftitanium, zirconium, niobium, tantalum and hafnium.

14. An article according to claim 6 wherein the getter materialcomprises zirconium metal and the barrier layer is zirconia.

15. An article according to claim 6 wherein the getter material iszirconium and the barrier layer is magnesia.

16. An article according to claim 15 wherein the barrier layer ofmagnesia includes a minor amount of silica.

17. An article intended to be used at high operating temperatures above1,000 C. having a core of the class consisting of molybdenum, tungsten,an alloy of molybdenum, and an alloy of tungsten, and a flame-sprayedcoating on said core of zirconium, a further flame-sprayed coatingthereon selected from the group consisting of zirconia and magnesia, anda sheath enclosing said coated core and comprising a material selectedfrom the group consisting of a platinum group metal and an alloy of atleast one platinum group metal.

i l l l

2. An article according to claim 1 wherein said core comprises an alloyof said molybdenum with 0.5 - 1.0 wt. percent of zirconium.
 3. Anarticle according to claim 1 wherein the barrier layer comprisesmagnesia.
 4. An article intended to be used at high operatingtemperatures above 1,000* C. comprising a core of a metal selected fromthe group consisting of niobium, tantalum, niobium-tantalum alloy,niobium-chromium alloy, tantalum-chromium alloy andniobium-tantalum-chromium alloy, a getter for oxygen on the coreconsisting essentially of a refractory coating, a sheath of a materialselected from the groUp consisting of a platinum group metal and analloy having at least one platinum group metal, and a barrier layerinterposed between said core and said sheath comprising a refractorycompound compatible with said core and said sheath at the operatingtemperature, whereby volatilization of oxides of said core material isreduced and said core material is prevented from forming with saidsheath an alloy having a melting point below said operating temperature.5. An article according to claim 4 further having an interfacial spacebetween the core and the sheath wherein the said space has a partialpressure of not more than 2 mm. of mercury.
 6. An article intended to beused at high operating temperatures above 1,000* C. comprising: (a) acore consisting essentially of a refractory metal selected from thegroup consisting of molybdenum, tungsten and a refractory alloy based onat least one of said metals; (b) a getter for oxygen on the coreconsisting essentially of a refractory coating; (c) a barrier layer onthe getter consisting essentially of a refractory compound selected fromthe group consisting of refractory carbides, silicides, borides,sulphides, nitrides and oxides, and (d) an outer sheath enclosing thecore, getter and barrier layers and consisting essentially of a materialselected from the group consisting of the platinum group metals and aplatinum group metal alloy, volatilization of the oxides of said corematerial thereby being prevented from forming with said outer sheath analloy of lower melting point than that of the sheath.
 7. An articleaccording to claim 6 having an interfacial space between the core andthe sheath wherein the said space has a partial pressure of not morethan 2 mm. of mercury.
 8. An article according to claim 7 wherein thegetter is a material selected from the group consisting of zirconium,tantalum, niobium, titanium, vanadium and hafnium.
 9. An articleaccording to claim 8 wherein the getter material is present inrelatively small quantities as dilute alloys of molybdenum or platinum.10. An article according to claim 6 wherein the getter is in the form ofa plasma or flame sprayed coating.
 11. An article according to claim 6wherein the getter is concentrated in an area which, in use, is notsubject to temperatures greater than 1,200* C. so that the getterabsorbs oxygen without contact with the barrier layer, thereby avoidingdecomposition of the sheath material.
 12. An article according to claim6 wherein the core material is molybdenum alloyed with minor quantitiesof at least one of the materials of the group consisting of titanium andzirconium.
 13. An article according to claim 6 wherein the core materialis alloyed with at least one of the metals selected from the groupconsisting of titanium, zirconium, niobium, tantalum and hafnium.
 14. Anarticle according to claim 6 wherein the getter material compriseszirconium metal and the barrier layer is zirconia.
 15. An articleaccording to claim 6 wherein the getter material is zirconium and thebarrier layer is magnesia.
 16. An article according to claim 15 whereinthe barrier layer of magnesia includes a minor amount of silica.
 17. Anarticle intended to be used at high operating temperatures above 1,000*C. having a core of the class consisting of molybdenum, tungsten, analloy of molybdenum, and an alloy of tungsten, and a flame-sprayedcoating on said core of zirconium, a further flame-sprayed coatingthereon selected from the group consisting of zirconia and magnesia, anda sheath enclosing said coated core and comprising a material selectedfrom the group consisting of a platinum group metal and an alloy of atleast one platinum group metal.